Chronic disease and decreased mobility associated with obesity in older (65+ yrs) adults emphasize the urgent need to identify obesity treatments in this population that are evidenced-based, effective in the long-term, and perhaps age-specific. The latest obesity treatment guidelines for all ages recommend an intensive intervention involving behavioral counseling, caloric restriction, and increased physical activity to achieve weight loss. However weight regain after treatment cessation is very common, especially in older adults in whom the currently recommended strategy of continued performance of high-volume structured exercise of 200-300 min/wk may be less feasible and effective. A key factor contributing to weight regain is the decrease in energy expenditure that occurs in response to weight loss, due mainly to a decline in non-exercise or spontaneous physical activity (SPA) that is the direct result of an increase in sedentary behavior (SB), or time spent sitting or reclining. Our pilot data show that decreases in SPA predict magnitude of weight regain. Since older adults spend 65-80% of their waking day sitting or prone, we hypothesize that intervening on SB will be a more effective method for preventing weight regain than the conventional approach of intervening on exercise behavior in this age group. The proposed research will test the efficacy of a novel, acceptable, behavioral intervention (SitLess) that focuses on increased awareness of SB employing accelerometry-based self-monitoring throughout the day. Our preliminary data show that this intervention improves weight loss during the intensive phase of treatment and prevents weight regain during a short (5-month) follow-up phase. The primary aim of this study is to determine whether addition of this intervention that targets SB to a conventional weight loss intervention that targets EX results in lower long-term reduction in body weight in older, obese adults. This will be accomplished with a 24-month trial in 225 obese (BMI=30-40 kg/m2) older (65-79 yrs) adults randomized to one of three treatments (n=75), all with dietary caloric restriction plus either: 1) moderate- intensity aerobic exercise (WL+EX); 2) intervening on SB (WL+SitLess); or 3) (WL+EX+SitLess). The primary hypothesis is that: WL+EX+SitLess will have lower 24-month body weight than either WL+EX or WL+SitLess. Secondary outcomes, including resting and physical activity energy expenditure, SB, dietary intake, body composition, physical fitness and function, cardiometabolic risk and intervention process measures, will also be examined. The results will provide the first randomized, controlled trial data on the efficacy o self-monitoring of a key contributor to the adaptive thermogenic response to weight loss. Increasing awareness of weight loss-induced reductions in SPA, and behavioral promotion of decreasing SB to counteract these reductions, may provide for a more effective, safe, non- pharmacologic, and sustainable strategy to reduce weight regain after weight loss in older adults.